It is well known in the art to prepare plastic and synthetic foam articles from polyurethanes, polyesters, epoxies, vinyl esters, polyamides, and the like, by combining two or more liquid organic reactive components, and thereafter injecting the reactive mixture into a mold cavity where the mixture cures and hardens into a finished plastic product. A particular problem associated with the handling, mixing, and dispensing of liquid reactive components is the tendency of the components to react rapidly with each other or upon exposure to the atmosphere, thereby causing the accumulation of undesirable reaction products within the mixing and dispensing device. These accumulations interfere with the thorough mixing and dosing of precise amounts of the reactive mixture, by restricting the mixing chamber and dispensing passageway at any or all points downstream from where the individual reactive components enter the mixing chamber. This problem is particularly severe when the dispensing device is used intermittently.
Several systems have been devised for purging unwanted reaction products from dispensing devices. U.S. Pat. No. 4,523,696 discloses the use of a reciprocating valve rod or plunger which, when located in a forward position, occupies the mixing chamber sealing the inlet ports of the liquid reactants, and, when located in a rearward position, opens the mixing chamber permitting the flow of liquid reactants into the mixing chamber. After the appropriate amount of liquid reactant mixture has been dispensed, the reciprocating valve rod is moved from its rearward to its forward position, thereby preventing the flow of individual reactants into the mixing chamber, and sweeping through the mixing chamber to mechanically expel the remaining liquid reactants therefrom. The patent discloses washing the reciprocating valve rod with one of the liquid components to prevent its binding during movement into and out from the mixing chamber, due to the accumulation of unwanted reaction products on the surface thereof. U.S. Pat. No. 4,471,887 additionally discloses the introduction of purging air into the mixing chamber when the reciprocating rod is in the rearward position.
U S. Pat. No. 4,285,446 discloses a multicomponent dispensing apparatus, wherein polyurethane foam reactants are combined in a mixing chamber, followed by the injection into the mixing chamber of a purging gas, such as pressurized air, for a predetermined time interval, thus purging the unwanted reaction products and readying the apparatus for dispensing another "shot" of the liquid reactant mixture.
Several purging processes utilize a solvent, which is admitted to the mixing chamber to dissolve and flush undesired reaction products and unreacted liquid components from the mixing chamber via the dispensing passageway or an exhaust port. U.S. Pat. Nos. 4,002,271; 4,426,023; and 4,516,694 disclose injecting a pressurized solvent directly into the mixing chamber or the dispensing port, to solubilize and flush accumulated deposits of unwanted reaction products, U.S. Pat. No. 4,440,320 discloses a rotary valve which oscillates between a first position in which the valve passageways communicate with channels supplying the liquid reactants, and a second position in which the valve passageways communicate with channels supplying a cleaning solution to the mixing chamber.
Finally, U.S. Pat. No. 4,033,481 discloses a liquid polyester resin and catalyst mixing and dispensing device, having a solvent port for admitting an air atomized stream of acetone or methylene chloride to the mixing chamber. The injection of atomized solvent is followed by a continuous stream of pressurized air, to dry the chamber and dispensing passageway. The solvent is atomized at a point at least eight to ten feet from the mixing chamber and conveyed to the chamber by means of a conduit, thereby insuring that non-atomized liquid does not enter the mixing chamber. The singular atomization, flushing, and drying cycle requires only a few seconds.